I see no reason not to go ahead and re-stock and perhaps also re-barrel your 670. No, it's not a valued pre '64 M70, but it's the same action as the post '64 and my 1966 '06 m70 is a very nice and very accurate rifle. It's not worth near what my pre '64 .270 F/W is, but who cares. It's good looking and shoots (with the original barrel) sub MOA. You didn't mention how yours shoots in '06 now, but if you're wanting something with a bit less recoil and as a crossover caliber, the baby '06 is a fine choice. I like the heavier 115-120gr loads for deer in mine, but that's simply personal preference. With 100gr loads available at 3300 FPS, it shoots flat enough for dual duties, for certain. I'd prefer a 22-24" standard profile over a heavy one for weight considerations, myself (especially for kids), but again, that's simply personal preference. What type of stock are you looking at putting on it?my 30-06 has developed a crack in the stock running forward on the left hand side, top of stock running the length of the action. i am gonna order another stock but would like maybe to change a few other things barrel, caliber. would this be worth the effort being its just a 670? i love the gun. since i bought my 300 my wife has really been pushing me to sell it but i just cant do it. im really thinkin 25-06 with a heavier barrel so i could pop yotes, deer, and when kids get older good first deer rifle. what other calibers would be an option on both ends of spectrum.
Other calibers I'd consider would be .270 or .280 below and .338/06 or .35 Whelen above. They are all excellent cartridges, with the .270 and .35W being far more mainstream and popular (read that less expensive ammo than their counterparts). For what it's worth, I own or have owned all those calibers and my favorite is the .35 Whelen.